1. Field of the Invention
This invention is directed to microelectronic devices and, in particular, to a method and apparatus for repairing aluminum nitride substrates for such devices.
2. Description of Related Art
Multi-layered electronic interconnect packages (substrates) for analog and/or digital circuits can be fabricated from a variety of materials such as alumina, glass, ceramic, epoxy-glass (i.e. FR4), and aluminum nitride (AlN) among others. In the choice of these materials, low dielectric constant is an important consideration. Additionally, it may be desirable for the material to exhibit a high thermal conductivity so that it can transfer heat away from the electrical device mounted on it, to its opposite surface where it can be more easily removed. Aluminum nitride exhibits both these desirable characteristics, as it has a dielectric constant lower than alumina (8.5), thermal conductivity about 6 to 8 times higher than alumina, and, more importantly, a coefficient of thermal expansion (TCE) closely matched to silicon (4.5 ppm/.degree. C.). All packaging materials are chosen for their low dielectric constants.
In the course of manufacturing, design errors, manufacturing defects or engineering design changes can be made which render the interconnect package useless. These errors include internal shorts in metallurgy within the layers of the substrate, as well as shorts on the surface due to manufacturing errors and/or other plating defects. Due to the high cost of fabricating interconnect packages in AlN, coupled with the fact that these errors are most often detected at testing after the substrates are completely fabricated (with maximum cost and time incurred), it is highly desirable to be able to rework the substrates and repair them.
Repair of AlN substrates can be divided into two categories: 1) repair or isolation of surface structures due to design errors such as via to pad connections, or via to ground plane shorts, engineering changes (ECs), or manufacturing defects such as extraneous plating shorting two features (i.e. pad to pad, pad to ground plane etc.); and 2) repair or isolation of sub-surface structures due to design errors or manufacturing errors (i.e. severing internal connections such as buried lines).
Surface repairs are performed by existing methods such as abrasive grinding, scraping and the like. These existing methods are unacceptable due to the typical size of the defect and proximity of the defect to other features. Also, abrasive grinding and/or scraping typically do not adequately isolate the defect but tend to imbed the metallic material into the substrate causing a high resistance short which is typically not an acceptable repair. Abrasive grinding also creates a great deal of debris which can be detrimental when found on a completed part.
Sub-surface repairs cannot be done by mechanical means due to the extremely tight tolerances, small sizes, and close proximity of other surface and/or buried structures. Sub-surface repairs can sometimes be accomplished on other substrate materials (i.e. alumina, glass ceramic, epoxy-glass etc.), by utilizing a finely focused laser beam. It has been found that when performed on an AlN substrate in air, the heat produced by the laser beam forms a plasma of conductive aluminum which tightly adheres to the substrate in the area of the repair. Additionally, the high temperature developed at the point of focus also locally changes the matrix of the aluminum nitride, causing metallic aluminum to be formed at the surface. This metallic surface layer provides an electrically conductive path at the repair area making such repairs unusable due to their low electrical resistance.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide a method for repairing AlN substrates which isolates the defect and minimizes high resistance shorts.
It is another object of the present invention to provide a method of repairing sub-surface defects in AlN substrates which does not produce a coating of conductive aluminum at the repair area.
A further object of the invention is to provide a method of removing unwanted surface metallurgy on AlN substrates which does not produce a coating of conductive aluminum at the repair area.
It is yet another object of the present invention to provide a novel work chamber for the repair of AlN substrates such that elemental aluminum is not produced at the repair area.
It is yet another object of the present invention to provide a method of using and an improved composition to passivate and seal the repair area from contamination.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.